>Pinnids are largely calcitic, as are pinnids, mytilids, ostreids, anomiids, limids, pectinids, propeamussiids and epitoniids (as well as brachiopods, echinoderms and crustaceans).<

Some limpets, nerites, and muricoideans (e.g., Ecphora) also have an outer calcitic layer.  Calcite occurs sporadically in other mollusk shells as well.  All mollusk shells have at least a little aragonite, in the muscle attachment areas.  Many have more extensive aragonitic inner layers and calcitic outer layers.  Most are entirely aragonite.

>It appears that nacre can be either aragonitic or calcitic.<

True nacre, i.e., mother of pearl, like the inner layer of modern Nautilus, various archaeogastropods, nuculids, pearl oysters, etc. is always aragonitic.  Pinnids and pterioideans have nacre inside and calcite outside.

The problem with drying out reflects the relative amounts of proteins versus carbonate in the shell.  Nacre has a relatively high level of proteins, as do taxa with flexible shells such as Pinnidae, Solemyidae, and Pteriidae, and so they may be prone to drying and cracking.  Although nacre is one of the strongest shell microstructures found in mollusks, once it does break it tends to fail catastrophically.  Other shell structure types may be easier to break, but breaks do not spread as easily.

>Calcite is far more stable over time<

Both calcite and aragonite are easily soluble in acid.  Aragonite is also unstable (on a geological time frame) and gradually changes into calcite, which usually involves crumbling away, hence the loss of original aragonitic shells from many fossil deposits.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa